RAPI client: Implement old instance creation request format

Commit 8a47b447 implemented instance creation in the RAPI client,
but it left out support for the old instance creation request format.
This patch now implements the old format as good as possible. This
will only be used when talking to clusters before Ganeti 2.1.3.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>